Field of the Disclosure
The present disclosure relates to an electrostatic capacitance type transducer, a drive method therefor, and a device using an electrostatic capacitance type transducer.
Description of the Related Art
Micro mechanical members manufactured using the micromachining technology have been able to be processed with micrometer precision, and various minute function elements have been implemented using the micro mechanical members. Electrostatic capacitance type transducers using such a technology are being studied as alternatives to ultrasonic transducers using piezoelectric elements. According to such electrostatic capacitance type transducers, acoustic waves, such as ultrasonic waves can be transmitted and received using vibration of a vibrating film. Especially in liquid, excellent broadband characteristics can be easily obtained using such electrostatic capacitance type transducers. The acoustic waves include sound waves, ultrasonic waves, and opto-acoustic waves. In the below description, the acoustic waves are represented by ultrasonic waves in some cases.
In the electrostatic capacitance type transducers, a conversion efficiency of an input voltage to an output sound pressure is proportional to the negative square of a distance between upper and lower electrodes. On the other hand, in a case where a large transmission sound pressure is output, it is necessary to set amplitude of a vibrating film large. In such a case, because the distance between the upper and lower electrodes varies with time, ultrasonic waves of harmonic waves having an input waveform are generated.
As a visualization technology using ultrasonic waves, there is a harmonic imaging method. The harmonic imaging method is a technique in which harmonic waves, but not a center frequency coming from a transducer, are received to generate an image. The harmonic waves are generated by nonlinearity of a density change and a sound pressure change in water and a living body tissue. When visualization according to the harmonic imaging method is performed using the electrostatic capacitance type transducer, it is predicted that harmonic waves from a tip of the transducer are mixed, because harmonic waves are generated by variation of the distance between the upper and lower electrodes with time. When harmonic waves are received, therefore, it becomes difficult to separate harmonic waves originated from a transmission sound source from harmonic waves originated from a medium to be measured. Therefore, it is desired to reduce harmonic waves included in a transmission sound pressure as far as possible. U.S. Pat. No. 6,461,299 discusses a method in which a waveform generator of a second harmonic wave is provided, and the second harmonic wave is added to a drive waveform for canceling a second harmonic wave generated as an ultrasonic wave.